Expression of the bZIP transcription factor gene Nrl in the developing nervous system.

Proteins of the Maf/Nrl subfamily of bZIP transcription factors are involved in the regulation of tissue-specific gene expression. The Nrl gene, initially identified from a subtracted retinal library, is expressed in all cell layers of the adult retina, including photoreceptors. The Nrl protein has high sequence homology with Maf proteins, binds to an AP-1 like sequence element, and in photoreceptors appears to be involved in regulating the expression of rhodopsin. In the present study, we investigated the expression of Nrl in the developing and adult mouse using in situ hybridization and RT-PCR. We demonstrate that beginning at embryonic day 12.5 Nrl is expressed throughout the developing central and peripheral nervous system, with the exception of the nasal epithelium. The spatial pattern of hybridization suggests that Nrl is transcribed in post-mitotic, differentiating neurons, the developing cephalic mesenchyme and lens. Nrl expression is downregulated postnatally in the brain, and becomes restricted to neocortex and brainstem in the adult. High levels of Nrl transcripts, however, persist in the mature photoreceptors and other retinal neurons. Our studies suggest a role for the Nrl protein in neuronal differentiation and in mature neurons of the adult retina.